It all depends on where you get the ticket from and what flight you're taking.
The first thing is basically the cost of the flight to and from one place to another, the staff they need for the flight, cost of all amenities, the meals as well as the cost they have to pay for landing & parking in the other airport. The total cost would be divided into the number of seats accordingly, with extra costs for the exclusive classes (biz & first) as they have much finer amenities, food and even the plating of their food costs more.
Then with the service, the awards the airline have... basically adding up to the status of the airline, they decide how much they can up the price while still having it at a competitive level with others.
Now if you're buying your ticket online, you'll notice that it's usually cheaper getting it online direct from the airline of choice than going through a website or through a travel agent. They might quote the price of the flight but after all the taxes, it's usually higher than a direct ticket from the airline as it already includes the fees for the airline staff, while other places will up it a bit more as they would have to make some money for themselves.
After you've got the ticket alone, you'll have to add the fuel tax and airport tax. You might have to check if you've covered the taxes for the other airport as some are included and some you'll have to pay at the airport itself.
So in the end you really have to look at where you'll be travelling to and from.